An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar space, typically defined as the region beyond the heliopause. It also refers to probes capable of reaching star systems other than the Sun's (capable of interstellar travel within a galaxy).
There are perhaps three areas to consider when examining interstellar probes:
- existing probes on interstellar trajectories (such as Voyager 1);
- plans for realistic interstellar probes (such as interstellar precursors); and
- more extraordinary plans using extreme cost, technology, or physics.
Even though termination shock happens as close as 80–100 AU, the maximum extent of influence of the Sun's gravitational field is thought to be the Hill/Roche sphere at around 230,000 AU (1.1 parsecs (3.6 light-years)). However, that is getting close to the nearest known star system, Alpha Centauri at 1.34 parsec (4.36 light years).
As of 2012, interstellar space is thought to lie beyond a magnetic region that begins at about 122 AU, as detected by Voyager 1.
Existing interstellar probes 
Five spacecraft (three active spacecraft) are currently on interstellar trajectories, unless they hit something after they stop communicating.
Functional spacecraft 
Voyager 1 (1977+) 
On 24 May 2005, a NASA press release expressed the consensus that Voyager 1 was now in the heliosheath. As of 2010 July 30, Voyager 1 is at a distance of around 10,619,000,000 miles (1.7090×1010 km) or 114.24 AU from the Sun and traveling outward at roughly 3.6 AU per year (17 km/s). Scientists believe the craft will reach the heliopause in 2015.
Voyager 2 (1977+) 
Voyager 2 passed the termination shock into the heliosheath on 2007 October 30. Unlike Voyager 1, it has a working plasma science instrument, which is enabling it to return data on the temperature and pressure of this region. As of 2010 July 30, Voyager 2 is at a distance of around 8,631,000,000 miles (1.3890×1010 km) or 93 AU from the Sun and traveling outward at roughly 3.263 AU per year (15.47 km/s). Voyager 2 is expected to keep transmitting into the 2020s.
New Horizons (2006+) 
Following its gravitational assist from Jupiter, New Horizons is on an interstellar trajectory via Pluto. By 2008 March 7, New Horizons was 9.37 AU from the Sun and traveling outward at 3.9 AU per year. It will, however, slow to an escape velocity of only 2.5 AU per year as it moves away from the Sun, so it will never catch up to either Voyager. As of early 2011, it was traveling at 3.356 AU/year (15.91 km/s) relative to the Sun.
Inactive missions 
- Pioneer 10 (1972–2003)
The last successful reception of telemetry from Pioneer 10 was on April 27, 2002, when it was at a distance of 80.22 AU. Traveling at about 2.54 AU/year (12 km/second).
- Pioneer 11 (1973–1995)
Routine mission operations for Pioneer 11 were stopped September 30, 1995, when it was 6.5 billion km (approx 43.4 AU) from Earth. Traveling at about 2.4 AU/year (11.4 km/second).
Probe debris 
New Horizons 3rd stage, a STAR-48 booster, is on a similar escape trajectory out of the Solar System as New Horizons, but will pass millions of kilometers from Pluto. It will cross Pluto's orbit in October 2015.
Trans-Neptunian probes at precursor distances 
In the early 2000s many new, relatively large planetary bodies were found beyond what was at the time planet Pluto, and with orbits extending hundreds of AU out past the heliosheath (90–1000 AU). The NASA probe New Horizons may explore this area after it performs its planned Pluto flyby in 2015 (Pluto's orbit ranges from about 29–49 AU). Some of these large objects past Pluto include 136199 Eris, 136108 Haumea, 136472 Makemake, and 90377 Sedna. Sedna comes as close as 76 AU, but travels out as far as 961 AU at aphelion, and minor planet (87269) 2000 OO67 goes out past 1060 AU at aphelion. Bodies like these have an impact on how the Solar System is understood, and traverse an area previously only in the domain of interstellar missions or precursors probes. After the discoveries, the area is also in the domain of interplanetary probes; some of the discovered bodies may become targets for exploration missions, an example of which is preliminary work on a probe to Haumea and its moons (at 35–51 AU). Probe mass, power source, and propulsion systems are key technology areas for this type of mission. In addition, a feature of this area is the gravitational lensing point[clarification needed] of the Sun at about 550 AU.
Proposed interstellar probes 
Missions intended to reach the interstellar medium.
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- Innovative Interstellar Explorer (2003+)
NASA proposal to send a 35 kg science payload out to at least 200 AU. It would achieve a top speed of 7.8 AU per year using a combination of a heavy lift rocket, Jupiter gravitational assistance, and an ion engine powered by standard radioisotope thermal generators. The probe suggested a launch in 2014 (to take advantage of Jupiter gravitational assist), to reach 200 AU around 2044.
- Realistic Interstellar Explorer & Interstellar Explorer (2000–2002)
Studies suggesting various technologies including Am-241-based RTG, optical communication (as opposed to radio), and low-power semi-autonomous electronics. Trajectory uses a Jupiter and Sun gravity assist to achieve 20 AU/year, allowing 1000 AU within 50 years, and a mission extension up to 20 thousand AU and 1000 years. Needed technology included advanced propulsion and solar shield for perihelion burn around the Sun. Solar thermal (STP), nuclear fission thermal (NTP), and nuclear fission pulse, as well as various RTG isotopes were examined. The studies also included recommendations for a solar probe (see also Solar Probe Plus), nuclear thermal technology, solar sail probe, 20 AU/year probe, and a long term vision of a 200 AU/year probe to the star Epsilon Eridani.
The "next step" interstellar probe in this study suggested a 5 megawatt fission reactor utilizing 16 metric tonnes of H2 propellant. Targeting a launch in the mid-21st century, it would accelerate to 200 AU/year over 4200 AU and reach the star Epsilon Eridani after 3400 years of travel in the year 5500 AD. However, this was a second-generation vision for a probe and the study acknowledged that even 20 AU/year might not be possible with then current (2002) technology. For comparison, the fastest probe at the time of the study was Voyager 1 at about 3.6 AU/year (17 km/s), relative to the Sun.
- Interstellar Probe (1999)
Interstellar Probe was a proposed solar sail propulsion spacecraft planned by NASA Jet Propulsion Laboratory. It was planned to reach as far as 200 AU within 10 years at a speed of 14 AU/year (about 70 km/s), and function up to 400+ AU. A critical technology for the mission is a large 1 g/m2 solar sail. The probe itself hoped to use an advanced RTG for electrical power, Ka-band radio for communication with Earth, Delta 2 for Earth launch, and a 25 kg instrument package using 20 watts.
- (Mewaldt et al., 1995)
Study for 200 kg probe traveling at 6 to 14 AU/ year using chemical and planetary or solar gravity assists.
- (Holzer et al., 1990)
Study for 1000 kg probe traveling at 10 AU/year using chemical and a solar gravity assist, to collect data out to 200 AU from the Sun.
- TAU mission (1987)
TAU mission (Thousand Astronomical Units) was a proposed nuclear electric rocket craft that used a 1 MW fission reactor and an ion drive with a burn time of about 10 years to reach a speed of 106 km/s (about 20 AU/year) to achieve a distance of 1000 AU in 50 years. The primary goal of the mission was to improve parallax measurements of the distances to stars inside and outside of our galaxy, with secondary goals being the study of the heliopause, measurements of conditions in the interstellar medium, and (via communications with Earth) tests of general relativity.
- Interstellar Precursor Mission (Jaffe, 1977–1979)
Proposed nuclear electric power/propulsion for a probe to 370 AU in 20 years and 1030 AU in 50 years from launch.
Interstellar concepts 
- Project Orion (1958–1965)
Project Orion was a proposed nuclear pulse propulsion craft that used fission or fusion bombs to apply motive force. The design was studied during the 1950s and 1960s in the United States of America, with one variant of the craft capable of interstellar travel.
- Bracewell probe (1960)
Interstellar communication via a probe, as opposed to sending an electromagnetic signal.
- Sanger Photon Rocket (1950s-1964)
- Enzmann Starship (1964/1973)
Proposed by 1964 and examined in an October 1973 issue of Analog, the Enzmann Starship proposed using a 12,000 ton ball of frozen deuterium to power thermonuclear powered pulse propulsion. About twice as long as the Empire State Building and assembled in-orbit, the spacecraft was part of a larger project preceded by large interstellar probes and telescopic observation of target star systems.
- Project Daedalus (1973–1978)
Project Daedalus was a proposed nuclear pulse propulsion craft that used inertial confinement fusion of small pellets within a magnetic field nozzle to provide motive force. The design was studied during the 1970s by the British Interplanetary Society, and was meant to flyby Barnard's Star in under a century from launch. Plans included mining Helium-3 from Jupiter and a pre-launch mass of over 50 thousand metric tonnes from orbit.
- Project Longshot (1987–1988)
Project Longshot was a proposed nuclear pulse propulsion craft that used inertial confinement fusion of small pellets within a magnetic field nozzle to provide motive force, in a manner similar to that of Project Daedalus. The design was studied during the 1990s by NASA and the US Naval Academy. The craft was designed to reach and study Alpha Centauri.
- Starwisp (1985)
Starwisp is a hypothetical unmanned interstellar probe design proposed by Robert L. Forward. It is propelled by a microwave sail, similar to a solar sail in concept, but powered by microwaves from an artificial source.
- Medusa (1990s)
Medusa was a proposed nuclear pulse propulsion craft that used fission or fusion bombs to provide propulsion, with a structure similar to a light sail used to derive motive force from the bombs' blast waves. The design was studied during the 1990s by the British Interplanetary Society as a means of interplanetary travel.
- Starseed launcher (1996)
Starseed launcher was concept for launching microgram interstellar probes at up to 1/3 light speed.
- AIMStar (1990s-2000s)
AIMStar was a proposed antimatter catalyzed nuclear pulse propulsion craft that used clouds of antiprotons to initiate fission and fusion within fuel pellets. A magnetic nozzle derived motive force from the resulting explosions. The design was studied during the 1990s by Penn State University. The craft was designed to reach a distance of 10,000 AU from the Sun in 50 years.
- Project Icarus (2009+)
Project Icarus is theoretical study for an interstellar probe and is being run under the guidance of the Tau Zero Foundation (TZF) and the British Interplanetary Society (BIS), and was motivated by Project Daedalus, a similar study that was conducted between 1973 and 1978 by the BIS. The project is planned to take five years and began on September 30, 2009
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Other interplanetary probes of interest 
Other probes of interest to suggested interstellar missions.
- NASA Dawn (launched 2007), Using solar powered xenon ion thrusters, achieving a velocity change of over 10 km/s 
- Solar Probe Plus, planned probe approaches Sun within about 8-9 solar radii. (Interstellar probes using Solar gravity assist need to survive Sun perihelion)
- Deep Space 1 (1999–2001), demonstrated ion engines and Ka-band radio communications.
- Ulysses (1990–2009), Out-Of-The-Ecliptic mission meant large velocity change of 15.4 km/s (IUS & Pam-S booster) and Jupiter gravity assist. Used RTG for power.
- IKAROS (2010) & NanoSail-D2, solar sail propulsion tests
See also 
- Selected programs
- NERVA (ending 1972), tested nuclear fission thermal rocket engine
- NASA Project Prometheus (ending 2006)
- Jupiter Icy Moons Orbiter (ending 2005), nuclear fission electric ion engine
- Breakthrough Propulsion Physics Program (1996–2002), evaluated advanced propulsion technology
- Interstellar Boundary Explorer (IBEX) (2008), space observatory that measured energetic neutral atoms from interstellar boundary.
- Outer Space
- List of nearest stars (nearest to Earth)
- Heliosphere & Interstellar Medium
- Kuiper belt (30-50 AU) & Oort cloud (2,000 to 200,000 AU)
- Local Interstellar Cloud (aka Local Fluff & 30 light years across) & Local Bubble (300 light years across) (1 light-year ≈ 63,241 AU)
- Examples of distance in AU
- Space travel
- "Interstellar Probe". Interstellar.jpl.nasa.gov. 2002-02-05. Retrieved 2010-10-22.
- Chebotarev, G.A. (1964), "Gravitational Spheres of the Major Planets, Moon and Sun", Soviet Astronomy 7 (5): 618–622, Bibcode:1964SvA.....7..618C, retrieved 2010-03-24
- NASA Voyager 1 Encounters New Region in Deep Space
- "Voyager Enters Solar System's Final Frontier". NASA. 2005-05-24.
- Spacecraft escaping the Solar System (Chris Peat, Heavens-Above GmbH)
- "VOYAGER AT 35: BREAK ON THROUGH TO THE OTHER SIDE".
- Joel Poncyb, Jordi Fontdecaba Baiga, Fred Feresinb, Vincent Martinota (Thales Alenia Space): A preliminary assessment of an orbiter in the Haumean system: How quickly can a planetary orbiter reach such a distant target? Acta Astronautica, vol. 68, issues 5-6, March–April 2011, p. 622-628 / special issue: Aosta 2009 Symposium. Available online 2010-06-07, retrieved on sciencedirect.com 2011-01-15
- Paul Gilster: Fast Orbiter to Haumea. Centauri Dreams—The News of the Tau Zero Foundation. July 14, 2009, retrieved January 15, 2011
- Paul Gilster: The FOCAL Mission: To the Sun’s Gravity Lens. Centauri Dreams—The News of the Tau Zero Foundation. August 18, 2006]
- "Innovative Interstellar Probe". Interstellarexplorer.jhuapl.edu. Retrieved 2010-10-22.
- Ralph L. McNutt, et all - Interstellar Explorer (2002) - Johns Hopkins University (.pdf)
- NASA's Interstellar Probe Mission (1999) (.pdf)
- "Preliminary scientific rationale for a voyage to a thousand astronomical units". Jet Propulsion Laboratory.
- "Tau - A Mission to a Thousand Astronomical Units". Jet Propulsion Laboratory.
- "Interstellar Precursor Mission" - Encyclopedia of Science (accessed November 1, 2010)
- Joel Davis - With anti-matter to the stars - New Scientist (Jun 24, 1989) (Google Books Link)
- Enzmann Starship
- Centari Dreams: A Note on the Enzmann Starship by Paul Gilster on April 1, 2007
- Ian Ridpath - Messages from the stars: communication and contact with extraterrestrial life (1978, Harper & Row, 241 pages) = Google Books 2010, Quote: "As long ago as 1964, Robert D. Enzmann of the Raytheon Corporation proposed an interstellar ark driven by eight nuclear pulse rockets. The living quarters of the starship, habitable by 200 people but with room for growth, ..."
- "Medusa: Nuclear explosive propulsion for interplanetary travel". Smithsonian/NASA ADS Physics Abstract Service.
- "Starseed/Launcher" - by Forrest Bishop (accessed October 28, 2010)
- Lewis, Raymond A; Meyer, Kirby; Smith, Gerald A; Howe, Steven D. "AIMStar: Antimatter Initiated Microfusion For Pre-cursor Interstellar Missions".
- Leonard David, "Futuristic interstellar space probe idea revisited", MSNBC, May 9, 2010.
- Stephen Ashworth FBIS, "Project Icarus - Son of Daedalus", Spaceflight, 454-455 (December 2009).
- "Dawn: Mission description". UCLA Space Physics Center. 2006-10-17. Retrieved 2007-09-28.
Further reading 
- NASA's Interstellar Probe Mission (1999) (.pdf)
- Leonard David - Reaching for interstellar flight (2003) - MSNBC (MSNBC Webpage)
- Ralph L. McNutt, et al. - A Realistic Interstellar Explorer (2000) - Johns Hopkins University (.pdf)
- McNutt, et al. - Radioisotope Electric Propulsion (2006) - NASA Glenn Research Center (includes Centaur orbiter mission)
- Scott W. Benson - Solar Power for Outer Planets Study (2007) - NASA Glenn Research Center (with SEP booster)